1. The Field of the Invention
The present invention relates to pressure difference creating devices which are used to control the rate of fluid flow in pipe sections and in particular to a Venturi of a new design which minimizes the pressure loss across it.
2. The Prior Art
Many devices which are used to control the rate of compressible fluid flow do so by inducing critical flow. The critical flow phenomena is defined in the art as the conditions which exist at the point where the fluid flow rate becomes independent of downstream pressure. These conditions occur because of the induced pressure discontinuity caused by sonic velocity having been reached in the device. When such devices are designed to minimize the pressure loss obtained in the device, they are known as Venturi tubes. Venturi tubes heretofor available have been long, generally difficult to manufacture requiring multipart construction and accurate assembly, difficult to install with standard couplings, and have an inherent one-way flow direction. Flow rate controlling devices, such as the venturi tube, are used commonly in the conveyance of steam in oil fields undergoing steam enhanced recovery treatment and also in the nuclear power industry to control steam in reactors.
A venturi is a shaped obstruction placed in a pipe to restrict the flow therethrough. The amount of flow is determined by the pressure difference across the obstruction, the properties of the flowing medium, and the specific geometry of the obstruction. When the mass flow rate of the medium no longer increases with increasing pressure drop, the flow has achieved a critical flow rate. The pressure drop necessary to obtain critical flow rate is extremely dependent on the geometry of the obstruction.
In many applications it is desirable to achieve critical flow for flow measurement or flow regulation. In some situations, a desired state is to achieve critical flow with as little pressure drop as possible, for example around 5-20% pressure drop. In other situations it is desirable to have a short venturi and/or even one which could handle flow from either direction.
In prior designs of Venturi tubes, most tubes were constructed in two or three parts, namely, 1) an inlet section, 2) a straight throat section, and 3) a diffuser section. The straight throat section, however, has not been found to be advantageous for critical flow devices. Some venturi tubes therefore omit the straight throat section and simply connect the inlet directly to the diffuser section. The straight throat section is often used in other applications, such as venturi scrubbers. An example of the straight throat section can be found in U.S. Pat. No. 1,850,030. Virtually all other venturi tubes have conical diffuser section of approximately 5.degree. cone angles, for example U.S. Pat. No. 4,174,734. Some devices have several segments of successively larger conical angles, as illustrated by U.S. Ser. No. 816,280 and U.S. Pat. No. 4,648,455. Venturi meters have a pressure sampling device in the throat and are designed to operate in sub-critical ranges. All prior inventions use a short inlet section and a long tapered diffuser section, regardless of whether or not a straight intermediate throat section is used. It will be noted that all of this prior art concerns venturi devices of relatively great length, in proportion to their diameter, and that they are suitable for flow in a single direction.